Re: [PATCH 3/6 v4] cfq-iosched: Introduce vdisktime and io weight for CFQ queue

[Gui Jianfeng <guijianfeng@cn.fujitsu.com>]

Vivek Goyal wrote:
> On Fri, Feb 18, 2011 at 02:04:18PM +0800, Gui Jianfeng wrote:
>> Vivek Goyal wrote:
>>> On Thu, Feb 10, 2011 at 03:47:16PM +0800, Gui Jianfeng wrote:
>>>
>>> [..]
>>>> +/*
>>>> + * The time when a CFQ queue is put onto a service tree is recoreded in
>>>> + * cfqq->reposition_time. Currently, we check the first priority CFQ queues
>>>> + * on each service tree, and select the workload type that contains the lowest
>>>> + * reposition_time CFQ queue among them.
>>>> + */
>>>>  static enum wl_type_t cfq_choose_wl(struct cfq_data *cfqd,
>>>>  				struct cfq_group *cfqg, enum wl_prio_t prio)
>>>>  {
>>>>  	struct cfq_entity *cfqe;
>>>> +	struct cfq_queue *cfqq;
>>>> +	unsigned long lowest_start_time;
>>>>  	int i;
>>>> -	bool key_valid = false;
>>>> -	unsigned long lowest_key = 0;
>>>> +	bool time_valid = false;
>>>>  	enum wl_type_t cur_best = SYNC_NOIDLE_WORKLOAD;
>>>>  
>>>> +	/*
>>>> +	 * TODO: We may take io priority and io class into account when
>>>> +	 * choosing a workload type. But for the time being just make use of
>>>> +	 * reposition_time only.
>>>> +	 */
>>>>  	for (i = 0; i <= SYNC_WORKLOAD; ++i) {
>>>> -		/* select the one with lowest rb_key */
>>>>  		cfqe = cfq_rb_first(service_tree_for(cfqg, prio, i));
>>>> -		if (cfqe &&
>>>> -		    (!key_valid || time_before(cfqe->rb_key, lowest_key))) {
>>>> -			lowest_key = cfqe->rb_key;
>>>> +		cfqq = cfqq_of_entity(cfqe);
>>>> +		if (cfqe && (!time_valid ||
>>>> +			     time_before(cfqq->reposition_time,
>>>> +					 lowest_start_time))) {
>>>> +			lowest_start_time = cfqq->reposition_time;
>>> Gui,
>>>
>>> Have you had a chance to run some mixed workloads in a group (some sync,
>>> some async and some sync-idle queues), and see how latency and throughput
>>> of sync-idle workload changes due to this "resposition_time" logic. I 
>>> just want to make sure that latency of sync-noidle workload does not
>>> go up as that's the workload that people care and gets noticed first.
>> Hi Vivek,
>>
>> I made a quick test by using fio. It seems the number changes little
>> between vanilla kernel and patched kernel.
>>
>>
>> Vanilla:    SYNC read            SYNC-NOIDLE read      ASYNC write  
>>          1. 23,640KB/s 5.40 ---- 6,696KB/s 19.07 ---- 50,142KB/s 128.00
>>          2. 24,459KB/s 5.22 ---- 6,775KB/s 18.86 ---- 47,349KB/s 129.89
>>          3. 25,929KB/s 4.93 ---- 7,378KB/s 17.32 ---- 32,350KB/s 131.88
>>
>> Patched:   SYNC read            SYNC-NOIDLE read      ASYNC write  
>>         1. 24,000KB/s 5.32 ---- 6,942KB/s 18.39 ---- 30,860KB/s 135.95
>>         2. 23,678KB/s 5.40 ---- 7,274KB/s 17.58 ---- 67,432KB/s 120.44
>>         3. 23,004KB/s 5.55 ---- 6,621KB/s 19.30 ---- 36,536KB/s 148.64
> 
> Hi Gui,
> 
> Do you also have latency numbers? I am especially interested max completion
> latencies of SYNC-NOIDLE workload.

Vivek,

Here some numbers about latency between vanilla and patched kernel.
I tested 4 times for each. It seems no latency regression happens.

Vanilla:
1. clat (msec): min=1, max=302, avg=18.19, stdev=39.80
2. clat (msec): min=1, max=201, avg=17.76, stdev=31.90
3. clat (msec): min=1, max=303, avg=18.64, stdev=41.30
4. clat (msec): min=1, max=370, avg=17.43, stdev=35.09

Patched:
1. clat (msec): min=1, max=176, avg=19.00, stdev=32.98
2. clat (msec): min=1, max=175, avg=17.75, stdev=32.41
3. clat (msec): min=1, max=191, avg=19.11, stdev=33.28
4. clat (msec): min=1, max=176, avg=17.11, stdev=32.99

Thanks,
Gui

> 
> Thanks
> Vivek
>